It is known that exposure of human or animal tissue to ionizing radiation will kill the cells thus exposed. This finds application in the treatment of pathological cells, for example. In order to treat tumors deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, resulting in prolonged recovery periods for the patient. It is, therefore, desirable to design a device for treating a patient with ionizing radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation which will result in the death of those cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumor from a number of directions, either simultaneously from multiple sources or multiple exposures from a single source. The intensity of radiation emanating from each direction is therefore less than would be required to actually destroy cells (although still sufficient to damage the cells), but where the radiation beams from the multiple directions converge, the intensity of radiation is sufficient to deliver a therapeutic dose. By providing radiation from multiple directions, the amount of radiation delivered to surrounding healthy cells can be minimized.
Of course it is also important that the radiation should be accurately targeted on the region that requires treatment. For this reason, patients are required to remain still for the duration of the therapy session, to minimize the risk of damage to healthy tissue surrounding the target region. However, some movement is inevitable, e.g. through breathing, or other involuntary movements.
A number of different techniques for the tracking of moving targets in radiotherapy are known. Many involve tracking the target by moving the leaves of the multi-leaf collimator (MLC), as described in U.S. Pat. Nos. 7,469,035 and 7,221,733. Others make use of a moveable patient support, such as in US patent application number 2008/0212737, or the Applicant's own US patent application number 2009/0168961, now U.S. Pat. No. 8,042,209. Tracking systems involving only motion of the couch or MLC leaves place high demands on the chosen device, and limit the degrees of freedom that can be used to track the target movement.
There has also been an attempt to use motion of the MLC leaves and the patient support in unison, as described in a paper by Podder et al (“Co-ordinated dynamics-based control of robotic couch and MLC-bank for feedforward radiation therapy”, The International Journal of Computer Assisted Radiology and Surgery, Vol. 2, pp. S106-S108). The “division of labour” between the two is based on the frequencies of the individual movements that make up the target trajectory. The movement of the target region is broken down into low- and high-frequency components, which can then be tracked by the patient couch and MLC leaves respectively. However, this approach is complex.